1. Field of Invention
The present invention relates to a cable length detection apparatus. More particularly, the present invention relates to an apparatus for measuring a length of a cable for transmitting an analog signal.
2. Description of Related Art
With the rapid development of information technology, computers and their peripherals have become very popular. Mice and keyboards are often used to control computers. Through the use of monitors or speakers, computer users can monitor the state of their computers.
The monitor usually uses three color signals, such as red, green and blue color signals, to make various other colors. The colors can be generated by combining these three color signals. Because general monitors are analog, the colors of their images are adjusted by controlling voltages such that every pixel can be displayed with continuous levels to represent vivid, photographic images.
In a computer system, video signals are transmitted to a monitor from a video adapter via a cable. However, the video signals are degraded by the impedance matching of the cable. If there is no compensation in the video signals, the images observed on the monitor are blurred. Contrarily, if the video signals are over-compensated, over-excited signals are formed, resulting in a reduction of monitor lifetime.
Moreover, when the length of the cable exceeds ten meters, the video signals are considerably lessened, thus degrading the quality of images and causing an operation error because of the unclear display. Specifically, various types of cables, such as a shielding twisted pair (STP) and a foil twisted pair (FTP), have distinct frequency responses. The video signals therefore exhibit different degradation phenomena when they are transmitted on various cables. The above reasons make it is difficult to use a simple signal compensation device in a fixed way to compensate the signal degradations occurring on cables of different types or different length.
In order to overcome the problems mentioned above, the prior art provides a method that uses a time difference between transmission and return of high-speed streams to calculate the length of cable. However, the conventional method is expensive and hard to perform. Moreover, the prior art provides another method that compresses an electrical signal into a UART line of the cable, and the UART line is typically used to transmit electrical signals for the keyboard or mouse.
However, this conventional method first has to stop the communications of other signals, and then performs the measurement of the length of the cable. For conditions with busy communications, such as continuous keyboard or mouse use, the conventional method substantially reduces communication bandwidth, and causes signal delay.